Document EP 2261513 discloses telescopic actuators comprising:                a body defining a cylindrical cavity of longitudinal axis;        a rod mounted to slide telescopically in the cylinder along the said axis;        a nut secured to the rod;        a lead screw mounted on the body to turn about the said axis and extend along the rod in order to collaborate with the nut in such a way that a rotation of the lead screw causes a telescopic movement of the rod in the body;        means for driving the rotation of the lead screw.        
In this type of actuator, the lead screw can be released axially to allow the rod to slide freely, for example if the means that drive the rotation of the lead screw jam. This release allows the rod to move freely under the action of the load coupled to the rod.
Thus, if such an actuator, which under normal circumstances is used to deploy and raise landing gear, develops a fault or loses its motive power, all that is then required is for the lead screw to be released and the landing gear will be able to deploy under the action of the weight of the landing gear which has a natural tendency to bring the landing gear into the deployed position.
However, in the aforementioned actuator, the action of the load tends to lenghten the actuator. Some landing gear actuating cylinders are mounted the other way around, namely such that the load has a tendency to shorten the actuator rather than to lengthen it.
The abovementioned actuator is then not capable of allowing the landing gear to move freely under its own self-weight, because this actuator cannot shorten, unless of course use is made of a reversible screw/nut connection, something which is not always possible. Further, even if it is, certain failure scenarios cause the rotating of the lead screw to jam, thus jamming the rod axially.